1. Field of Invention
The present invention relates, generally, to high-pressure fuel systems for internal combustion engines and, more specifically, to a tappet assembly for use in an internal combustion engine high-pressure fuel system.
2. Description of the Related Art
Conventional internal combustion engines typically include one or more camshafts in rotational communication with a crankshaft supported in a block, one or more intake and exhaust valves driven by the camshafts and supported in a cylinder head, and one or more pistons driven by the crankshaft and supported for reciprocal movement within cylinders of the block. The pistons and valves cooperate to regulate the flow and exchange of gasses in and out of the cylinders of the block so as to effect a complete thermodynamic cycle in operation. To that end, a predetermined mixture of air and fuel is compressed in the cylinders by the pistons, is ignited, and combusts; thereby transferring energy to the crankshaft via the piston. The mixture of air and fuel can be achieved in a number of different ways, depending on the specific configuration of the engine.
Irrespective of the specific configuration of the engine, contemporary engine fuel systems typically include a pump adapted to pressurize fuel from a source, such as a fuel tank, and direct pressurized fuel to one or more fuel injectors selectively driven by an electronic controller so as to atomize the pressurized fuel, which mixes with air and is subsequently used to effect combustion in the cylinders of the engine.
In so-called “port fuel injection” (PFI) gasoline fuel systems, the fuel injectors are arranged up-stream of the intake valves of the cylinder head, are typically attached to an intake manifold, and are used to direct atomized fuel toward the intake valves which mixes with air traveling through the intake manifold and is subsequently drawn into the cylinders. In conventional PFI gasoline fuel systems, a relatively low fuel pressure of 4 bar (58 psi) is typically required at the fuel injectors. Because of the relatively low pressure demand, the pump of a PFI gasoline fuel system is typically driven with an electric motor.
In order to increase the efficiency and fuel economy of modern internal combustion engines, the current trend in the art involves so-called “direct injection” (DI) fuel system technology, in which the fuel injectors admit atomized fuel directly into the cylinder of the block (rather than up-stream of the intake valves) so as to effect improved control and timing of the thermodynamic cycle of the engine. To this end, modern gasoline DI fuel systems operate at a relatively high fuel pressure, for example 200 bar (2900 psi). Because of the relatively high pressure demand, DI gasoline fuel systems typically utilize a high-pressure fuel pump assembly that is mechanically driven by a rotational movement of a prime mover of the engine, such as one of the camshafts. Thus, the same camshaft used to regulate the valves in the cylinder head is frequently also used to drive the high-pressure fuel pump assembly in DI fuel systems. To this end, one of the camshafts typically includes an additional lobe that cooperates with a tappet supported in a housing to translate rotational movement of the camshaft lobe into linear movement of the high-pressure fuel pump assembly.
The high-pressure fuel pump assembly is typically operatively attached to the housing, such as with removable fasteners. The housing may be formed as a discrete component or realized as a part of the cylinder head, and includes a tappet cylinder in which the tappet is supported for reciprocating movement.
The tappet typically includes a bearing which engages the lobe of the camshaft, and a body supporting the bearing and disposed force-translating relationship with the high-pressure fuel pump assembly. The high-pressure fuel pump assembly typically includes a spring-loaded piston which is pre-loaded against the tappet body when the high-pressure fuel pump assembly is attached to the housing. Thus, rotational movement of the lobe of the camshaft moves the tappet along the tappet cylinder of the housing which, in turn, translates force to the piston of the high-pressure fuel pump assembly so as to pressurize fuel. As the lobe of the camshaft continues to rotate, potential energy stored in the spring-loaded piston of the high-pressure fuel pump assembly urges the tappet back down the tappet cylinder so as to ensure engagement between the bearing of the tappet and the lobe of the camshaft.
During engine operation, and particularly at high engine rotational speeds, close tolerance must be maintained between the lobe of the camshaft, the tappet, and the piston of the high-pressure fuel pump assembly. Excessive tolerance may result in poor performance as well as increased wear, which leads to significantly decreased component life. Thus, it will be appreciated that it is important to maintain tolerances between the lobe of the camshaft, the tappet, and the piston of the high-pressure fuel pump assembly under varying engine operating conditions, such as engine rotational speed or operating temperature.
Each of the components of an internal combustion engine high-pressure fuel system of the type described above must cooperate to effectively translate movement from the lobe of the camshaft so as to operate the high-pressure fuel pump assembly at a variety of engine rotational speeds and operating temperatures and, at the same time, maintain correct tolerances so as to ensure proper performance. In addition, each of the components must be designed not only to facilitate improved performance and efficiency, but also so as to reduce the cost and complexity of manufacturing and assembling the fuel system, as well as reduce wear in operation. While internal combustion engine high-pressure fuel systems known in the related art have generally performed well for their intended purpose, there remains a need in the art for a high-pressure fuel system that has superior operational characteristics, and, at the same time, reduces the cost and complexity of manufacturing the components of the system.